1. Field of the Invention
The present invention relates to an image (an imagery data) transmission system and more particularly to an image transmission system incorporated into a monitoring system for a plant or a security system for a building, having a large number of sites to be monitored or observed so that the large amount of image data is transmitted between a monitor room and the sites being monitored or observed.
2. Description of the Related Art
Monitoring systems for various kinds of plants, security systems and the like employ an image transmission equipment. Typically, such image transmission equipment employs a plurality of television cameras situated at the sites to be monitored or observed. Image representation signal generated by a television camera are digitized by an encoder which employs both bandwidth compression and encoding the encoded signals are time-division multiplexed with digitized signals from other cameras for transmission over a transmission line.
In a monitoring room located at a control site remote from the siting equipped with the cameras, a demultiplexer demultiplexes the encoded image signals transmitted from the sites being monitored. The demultiplexed image signal is decoded by a decoder and an image is displayed on a corresponding monitor television set, so that an operator in the monitoring room can observe current conditions in the plural sites within a plant or building. In such an image transmission system, it is necessary to transmit image information from the cameras in real-time. In addition, it is preferable that malfunction of plant elements and unusual occurrences within a building observed via the television monitor.
Consider the case of a monitoring system where television cameras are provided at 50 sites of a plant to be monitored. If an image signal from each camera is converted into a digital signal encoded with 8 bits and this digital signal is transmitted at a base frequency of 4 MHz, the transmission rate required of the transmission line becomes as follows: EQU 50.times.4 MHz.times.2.times.8 bits=3.2 Gbps
wherein the sampling rate for encoding the image signal depends on Shannon's sampling theorem.
At present, it is very difficult to realize a transmission line having such a bandwidth as calculated above. At least it can be said that it is almost impossible to realize such a transmission line commercially. On the other hand, the number 50 is not an unusually large number of sites to be monitored in a typical plant. Further, in a security system for a large building, the number of sites to be observed will typically be much larger.